The invention describes a method for engineering interfaces between metals and the oxides of metals, and more particularly to a process to increase the strength and stability of an oxide-metal interface.
Metals deposited on an oxide surface generally do not wet the surface because the metal-to-oxide interaction is relatively weak. At room temperature, metal islands form on an oxide surface and grow until they merge into a metal overlayer. The morphology of the resulting metal layer is thus quite variable within several nanometers (nm) of the oxide surface, and the resulting interface is also quite non-uniform and weak. To strengthen the interface, reactive metals are sometimes added, such as brazing compounds. However, it is sometimes undesirable for such metals to be added because they often react with the oxide to form an intermediate layer which is poorly defined and contrary to the desired structure, such as within a microelectronics device.
One example of an application incorporating metal-to-oxide interactions includes prototypes being tested for the next generation of computer memory, magnetic random access memory (MRAM), which contain junction tunneling devices. For example, aluminum can be deposited on a magnetic layer, such as cobalt doped with iron. Subsequent oxidation produces an ultrathin (0.5 nm to less than 2 nm) layer of aluminum oxide. Another magnetic layer must then be deposited on the oxide. It is essential for MRAM applications that this interface be uniform, reproducible, and stable. However, the cobalt-oxide interaction is sufficiently weak to prevent wetting and the weak interfaces which result can be a cause of failure, that is, that the cobalt no longer adequately adheres to the oxide surface. In addition, because the metal grows in a three dimensional fashion on the oxide with islands nucleated at defects such as steps and impurities, there can be a large stochastic variability in structure and consequently a poor magnetization and uniformity in the tunneling resistance produced by the oxide film.
Another area where engineered interfaces can be important is heterogeneous catalysis. Metal particles are produced on a support material, commonly alumina. The dispersion (size) and shapes of the metal particles are important to the rate of the desired chemical reaction. In turn, the shape of the particles depends on the interfacial energy between the metal and the oxide, and can in principle vary from flat two dimensional islands (if strong interactions are present) to three dimensional amorphous or faceted objects having minimal contact to the oxide (if the interaction is very weak). The ability to control this shape by interfacial engineering of the adhesion energy would provide an additional tool for catalyst design.
The sealing of metals to ceramics is yet another application in which adhesion properties are important. In some applications, the use of added reactive metals is undesirable and producing good seals without them requires another method to achieve wetting and interface strength.
Needed is an alternate technique of interfacial engineering that can cause a metal to wet and which allows control or increase of oxide-metal adhesion.